This invention relates generally to gas turbines and in particular to methods and means for operating a gas turbine that does not have means of directly determining turbine power output (e.g., does not use generator load feedback) to maintain a substantially constant fuel-air ratio during changing load conditions.
Gas turbines coupled to electric generators are commonly used in power generation service. As gas turbines are capable of being quickly started and brought up to speed for operational loading, such units are generally considered more effective to use to handle grid peak loads (that is power demand spikes above a more constant grid base load) than steam turbine systems. Thus gas turbines are commonly used in a changing-load environment, in which they must respond to numerous increases and decreases in electric power demand.
It is desirable to optimize operation of a gas turbine in order to reduce undesirable emissions from the combustion process in the turbine. For example, the gas turbine combustion process results in generation of, among other things, nitrogen oxides (NO.sub.x), unburnt hydrocarbons, and carbon monoxide (CO). Such undesirable emissions can be minimized through control of the turbine's reaction zone temperature; this temperature in turn is well correlated with the fuel-air ratio (FAR) of the combustible mixture being fed into the combustion chamber of the turbine. Controlling the turbine's FAR thus effects overall operation of the combustor and is an important factor in optimizing turbine performance.
Current gas turbine control systems typically employ a decentralized control strategy in which fuel supply to the turbine and air supply to the turbine are controlled by reference to different measured turbine performance parameters. For example, in a typical gas turbine controller fuel supply to the turbine is controlled primarily via a feedback loop that seeks to match turbine power output with the electrical load demand on the generator driven by the turbine. This feedback is typically through turbine speed, with a speed error signal (that is variation of the measured turbine speed with a reference (or set point) value) being processed to increase or decrease fuel supply to the turbine as appropriate. Air supply to the turbine in such a system is typically determined by the compressor inlet geometries which are controlled based on the error between actual turbine exhaust temperature and a reference temperature value; the compressor inlet guide vanes are positioned to increase or decrease air flow into the turbine as necessary to obtain the optimal exhaust temperature. Thus, a change in load on the turbine results in an immediate change in fuel flow to the turbine, leading to a change in the power output and exhaust temperature, which situation results in the controller commanding the inlet guide vanes to open to increase air flow to the turbine.
Control of the gas turbine is further complicated when the gas turbine drive shaft is coupled to a generator that is also receiving rotational energy input from another source, such as a steam turbine. In this arrangement, the generator power output cannot be used as an estimate of the mechanical power output of the gas turbine alone.
In gas turbine control systems commonly in use, efforts to reduce FAR variations resulting from the lag between fuel supply control and air supply control (that is, fuel leads air in the current control system) by simply changing control constants and gains have generally been unsuccessful. For example, attempts to reduce the lag by increasing the gain on the air flow loop, reducing the gain on the fuel loop, or both, generally puts the control system into a response region which is highly oscillatory and closer to the instability region.
It is desirable to have a control system that controls fuel and air flow to the turbine in a coordinated manner during transient and steady state operations so as to maintain a substantially constant FAR.